1. Field of the Invention
The field of the invention relates to very small aperture terminals and systems for communicating with a satellite.
2. Discussion of the Background
VSATs are used for bi-directional satellite communication systems such as the system depicted FIG. 7. As to the transmit direction, FIG. 7 depicts a conventional VSAT having an ODU (outdoor unit) 18 and an IDU (indoor unit) 12 which generally includes a modem and power amplifier for transmitting and amplifying an L-band signal along transmission line 14.
In this conventional system, transmission line 14 connects to an up-converter 16 housed within an ODU 18, where the up-converter 16 up-converts the L-band signal to a microwave frequency signal and also amplifies the microwave frequency signal. In order for the up-converter 16 to up-convert the L-band signal to a specific microwave frequency, the IDU 12 may also transmit a reference frequency, usually at 10 MHz, along with the L-band signal to the up-converter 16.
The specific microwave frequency may be in the Ku-band, and this transmission is transmitted via waveguide 20, and a feed horn assembly and parabolic reflector (not shown), to a satellite 22. The satellite 22 then transmits or repeats the transmission signal to a hub 24 where the transmission signal is received and used for various purposes such as voice, interne or network access.
Generally, the transmission signal from the 18 to the satellite 22 operates according to a TDMA (Time Division Multiple Access) network protocol at a specific operating or transmission frequency of the ODU 18.
As to the receive direction, or data reception at the IDU 12, the hub 24 transmits a transmission signal via the satellite 22 back to the waveguide 20 where the transmission signal is converted by a low noise block converter 26. The low-noise block converter 26 converts the signal back into an L-band signal for transmission along the transmission line 28 back to the IDU 12.
The transmission lines 14 and 28 are usually a coaxial cable, such as industry standard RG-59/U or RG-6/U. The specific type of transmission line is chosen based upon various transmission requirements such as power, frequency, distance and voltage.
The computer terminal 30 may act as a data transmitter and receiver via a connection to the modem. The transmission signal generally includes data supplied by the computer terminal 30 or hub 24 (depending on whether it's an upstream or downstream transmission) transmitted along a carrier signal at a transmission frequency.
In order to configure the IDU 12 and the ODU 18 for proper transmission to the satellite 22, the IDU 12 and the ODU 18 may first be commissioned and calibrated. In commissioning and calibrating a conventional IDU 12 and ODU 18, both first need to be installed and fixedly secured at an operating location. Further, after the IDU 12 and ODU 18 are fixedly secured at the operating location, the transmission lines 14 and 28 are customized to be suitable for each different operating location. The customization may include cutting the transmission lines 14 and 28 to desired lengths suitable for the operating location and/or routing and fixedly securing the transmission lines in a manner suitable for the operating location.
In this conventional system, it is important to customize the transmission lines after securing the IDU 12 and ODU 18 because the electrical response of the IDU 12 and the ODU 18 depend on the transmission losses incurred along the transmission lines 14 and 28. Depending upon the operating location, transmission lines 14 and 28 may vary in length by hundreds of feet, where a substantial amount of transmission power may be lost. The transmission losses across transmission lines 14 and 28 are also affected by environmental noise and other electromagnetic wave generating devices such as wireless networks and cellular transmissions. Also of importance, any bends or kinks in the transmission lines 14 and 28 affect transmission losses. Accordingly, in this conventional system it is important to customize the transmission lines after securing the IDU 12 and ODU 18 to maximize the communication efficiency of the VSAT.
Once the IDU 12, ODU 18, and transmission lines 14 and 28 are fixedly secured, the power amplification of the IDU 12 can be configured. The power amplification is generally set so as to limit operation of the modem to a linear gain response. That is, operation of the modem is preferably set below a saturation level, where if gain is further increased, the modem may be damaged or the transmitted signal may be distorted. Additionally, in a gain range approaching saturation, also known as a non-linear response range, the output transmission power will stop increasing as gain is increased.
The linear gain response of the power amplification of the IDU 12 and up-converter 16, however, is frequency dependent. The transmission loss across transmission lines 14 and 28 is also frequency dependent. In order to test and monitor the transmission loss and the linear gain response in a conventional system, a test frequency may be used for communicating to the hub 24 via satellite 22. This test frequency may be several hundred megahertz away from an operating transmission frequency.
In the conventional system, the hub 24 will be in communication with a technician manually configuring IDU 12 so as to convey the necessary transmission details to ensure the maximum power amplification of the transmission signal by the IDU 12, and the transmission signal is received at the hub 24 with an adequate carrier to noise ratio. Namely, the hub 24 needs to receive a minimum threshold signal of a carrier to noise ratio (a requirement supplied by the hub 24) for proper transmission of the data by the transmission signal. The carrier to noise ratio of the transmission signal is frequency and power dependent.
Since there is a plurality of VSATs attempting to connect to hub 24 via satellite 22, the test frequency used by conventional VSATs for determining the transmission losses and the power requirements cannot be the same or near the normal operating transmission frequency of the other VSATs because the output characteristics of the IDU 12 and ODU 18 are yet to be determined. Thus, in the conventional system, if a signal is sent to the satellite with an incorrect polarization or incorrect carrier to noise ratio, the signal may disrupt other VSAT transmissions on the same network.
Thus, in the conventional VSAT, the test frequency serves may be used to estimate the characteristics of the IDU 12 and the ODU 18 operating at the actual transmission frequency. Once, the characteristics have been determined by the hub 24, and the IDU 12 and the ODU 18 have been properly configured, the ODU 18 is set to operate at the transmission frequency.
However, in the conventional system as mentioned above, the power amplifier of the IDU 12 may not be calibrated at the most efficient level possible because a test frequency was used instead of the actual operating frequency of the ODU 18.